1. Field of the Invention
The present invention relates to an elastic wave filter provided with an elastic wave resonator such as a surface acoustic wave (SAW) resonator or the like.
2. Description of the Related Art
For the elastic wave filter installed on a mobile terminal such as a cellular phone or the like and performing distinction of frequency signals, a ladder type filter 100 having a configuration such that a constant-K circuit taking a series arm 101 and a parallel arm 102 for a base section as shown in FIG. 9 is formed, and plural sets of the base section 103 of the constant-K circuit are connected in series is adopted. Then, it becomes possible to allow a signal having a specific frequency to pass through by making a resonance frequency of the series arm 101 and an anti-resonance frequency of the parallel arm 102 in the base section 103 nearly consistent with each other.
For the series arm 101 and the parallel arm 102 like these, SAW resonators or the like which are easy to downsize and capable of simultaneously forming plural resonators by a photolithography technology are often adopted. In such a case, for instance, as shown diagrammatically in FIG. 9, the base section 103 of the constant-K circuit is modularized by simultaneously forming these resonators 101 and 102 above a common chip or the like to configure a ladder type filter 100 by connecting plural resonator modules 110.
FIG. 10 is a plan view showing a configuration of the conventional resonator module 110, and the base sections 103a and 103b worthy of two sections shown in FIG. 9 are installed in the resonator module 110. Note that the following explanation will be made in taking the upper side facing the drawing as a front side and taking the lower side as a rear side. The resonator module 110 shown in FIG. 10 is patterned in a manner that three rectangular electrode units 111a, 111b and 121 including an interdigital transducer (IDT) electrode and reflectors provided on both sides thereof are aligned parallel in front and behind and these are connected with a signal line 132 to each other. The SAW resonator is configured with these electrode units 111a, 111b and 121, and an under layered piezoelectric substrate 131. These SAW resonators serve as the series arms 101a and 101b of the base sections 103a and 103b of the constant-K circuit shown in FIG. 9 by connecting the front side electrode unit 111a to an input port 136 and the rear side electrode unit 111b to an output port 137. Also by grounding the middle stage side electrode unit 121, the SAW resonator in question serves as the parallel arm 102 of the constant-K circuit. Note that the parallel arm 102 shown on the middle stage in FIG. 10 serves as a roll worthy of two parallel arms 102a and 102b corresponding to the respective base sections 103a and 103b shown in FIG. 9 with this one arm.
The resonator module 110 shown in FIG. 10 is formed above the piezoelectric substrate 131 of about 1.0 mm×1.5 mm for instance. Although downsizing has been tried up to now, empty spaces S1 through S4 surrounded by an alternate long and short dash line in the drawing are still reserved, and therefore there is a space capable of further downsizing. For instance, the empty spaces S1 and S2 are formed by largely bulged out by a wide signal line 132 connecting the front side series arm 101a and the rear side series arm 101b toward left of an area where the respective electrode units 111a, 111b and 121 are arranged. The empty spaces S3 and S4 are formed by a difference in size (difference in width in the present example) of the respective electrode units 111a, 111b and 121 between the series arms 101a, 101b and the series arm 101. The whole resonator modules are covered with, for instance, a metal cover, but the illustration thereof is omitted for the sake of convenience.
Here, “133a through 133d” in FIG. 10 are alignment marks for direction recognition of the resonator module 110, “134” is a patterning finish-recognition pattern and “135” is an identification code for identifying an item number or the like of the resonator module 110, and these are provided in the above-described empty spaces S1 through S4. The empty spaces S1 through S4 are used also as an area to provide information for managing the resonator module 110 in this manner, but there are many unnecessary spaces where are not used completely. Therefore, it is necessary to minimize such an unnecessary space as much as possible for further downsizing of the resonator module 110.
The empty spaces S1 through S4 like these are seemed to be cancelled by narrowing the signal line 132 or making the size of the respective electrode units 111a, 111b and 121 uniform. However, the width of the signal line 132 is determined from the viewpoint of reducing an ohmic loss (resistance loss) of the signal, and the size of the respective electrode units 111a, 111b and 121 serve as a design parameter regarding characteristics (such as resonance frequency or the like) of the series arms 101a, 101b and the parallel arm 102. Therefore, it is impossible to determine any of these values from the viewpoint of canceling the empty spaces S1 through S4.
In addition, as a problem separated from the above-described empty spaces S1 through S4, there comes up another problem in a manner that when the signal line 132 having a large area shown in FIG. 10 is adopted from the viewpoint of preventing the ohmic loss, parasitic capacitance of the signal line 132 also increases, which leads to deterioration of a pass band of the filter.
Note that in Patent Document 1, a ladder type filter omitting a space to dispose an extra signal line by using a reflector of a parallel arm resonator as a signal line connecting between the IDT electrodes of a series arm resonator is described. However, electrode fingers provided in a large number to a reflector are very thin of about several μm for instance, and the use of such a thin electrode finger as the signal line may lead to a large ohmic loss and it is not realistic. On the other hand, since the area of the whole reflector is large even when compared with a conventional signal line, the parasitic capacitance becomes large and the bass band of the filter is deteriorated.
Japanese Patent Laid-open No. 2000-31780: 0023th paragraph, FIG. 1